Retentive pads which not only retain a large volume of liquid, but are capable of self-metering have long been desired for lubricating and marking applications. Modern technology in these fields has particularly sought pads which meet these criteria and additionally are sufficiently economical in manufacture that they can be discarded after use. Obviously, long-life pads with lifetimes of 60,000 to 200,000 impressions are highly desirable. Ideally, this type of lifetime should be almost unrestrictive for the type of liquid and the nature of the pad material in order to have the widest application for a myriad of uses.
There have been attempts to produce pads which are self-metering and retentive.
U.S. Pat. No. 3,253,542 to McDonough produced an inked roller which was a substantial improvement over previous rollers and gave thousands of ink transfer operations. However, the process requires a compatible plasticizer for the resin to form a plastisol. Salt is added to the plastisol prior to the formation of molding and later leached out. The molding operation was particularly time consuming. Also, the plastisol must be compatible with the filler liquid, in this case ink vehicle; and this severely limits wide applicability.
U.S. Pat. No. 3,971,315 to Hansen discloses a two layer structure which provides a superior reservoir system. However, compatibility between the thermoplastic resin, plasticizer for plastisol formation and ink which is dispersed in the macroporous reservoir layer is necessary and limits wide applicability. While greater fluid retention is available, surface metering is not available here.
Therefore, it is an object of the present invention to produce a liquid retentive pad that can largely be produced from a wide variety of materials.
Furthermore, it is an object of the present invention to produce a pad which can be used with a wide variety of liquids tailored for specific uses and not restricted by the compositional requirements of the pad.
It is an additional object to produce a self-metering retentive pad in an efficient, rapid manner.
It is also an object of the present invention to produce multi-layered ink printers suitable for both low and high centrifugal or surface speed printing operations.
Yet another object of the present invention is to produce extrudates from extrusion molding processing which contain a high proportion of void formers.
It is also an object of the present invention to produce a self-metering retentive pad in a manner which results in a consistent density from part to part.